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The hole can be enlarged 1/64" in the direction necessary so it centers better with the valve port. Slightly more is ok but 1/32" is too large. When the gasket is compressed under transfer sleeve it will expand to seal tight with the slightly larger hole. Everything will center and seal well once the breech is installed.

In the photo above a tapered round file is used to enlarge the hole. The file should be held so it's centered in the opposite hole on the bottom of the tube. With light pressure applied in the direction needed, keep the file positioned so you don't enlarge the opposite hole. Check your progress every few strokes so you don't remove too much metal. The thicker part of the file should be larger than the hole so it keeps the hole round as it gets further in. 

In the photo below a piece of 320 grit wet/dry sandpaper is rolled into a stiff tube and used to ream the hole smooth and round. The paper should be spun so it expands to fit the hole, then change directions as needed so it stays snug enough to work but not tight enough to bind. It generally only takes a minute or two to achieved a smooth finished look as shown in the last photo.

The Discovery tube shown above has the hole for the valve port off-center towards the front edge of the screw holes. The blue tape runs straight along the front and rear edges of the screw holes to show the misalignment.  The Magnum Valve is clearly not the issue, the valve port is centered with the valve screws as it should be to align with the breech. This tube was received from Crosman in December 2018, older Discovery tubes generally have the hole too far to one side.

  The hole for the valve port in some tubes is not centered where it's supposed to be. This causes the valve port to be out of alignment with the tube. In most cases this isn't a significant problem. With a steel breech the set screw for the barrel should be loose during assembly. This allows the barrel to twist and slide slightly so the transfer sleeve can tilt to align with the valve port. The gasket allows the sleeve to tilt and still get a good seal.

 If the misalignment is severe it can affect performance and cause breech parts to fit poorly, leak and bind. This problem can be random with any tube but is often consistent with certain model variants or manufacturing periods. At the time this was written, most Discovery and 2240 tubes had been bad for several years. A simple fix is explained below.

The finished result with one of our Magnum Valves is shown below. The valve port is perfectly centered and seals tight.

Though the hole in this tube was not centered to begin with, it did align with a new Discovery valve. It didn't with an older valve or the Magnum Valve. The inconsistency of Crosman's manufacturing is the cause of this issue.

The port on the Magnum Valve is round but because it comes out of the top at a sharp angle the opening is oval. When the hole is deburred it tends to make the opening look off-center towards the back of the tube. This ls partly an optical illusion but the port does lean slightly in that direction because of the oval shape. So long as the hole in your tube is relatively centered it will align and seal perfectly.

The gasket gets compressed between the sleeve and the valve with a lot of pressure, it will seal very tight. The breech holds the sleeve centered over the valve port. The sleeve inserts into the gasket to prevent it from obstructing the port when compressed. The rubber gasket allows the transfer sleeve to tilt slightly and still seal well. It's a much better transfer port design then many give Crosman credit for. Even with a sloppily manufactured tube and valve it seals and aligns when all the parts are assembled. Adjusting the alignment of the hole in the tube assures the best possible performance, breech fit and function.